(Real Simple) -- Feel as if you're about to snap? Learn what causes stress, along with proven strategies for controlling it.

If you're going through a stressful period, fight the urge to snack endlessly and try to exercise regularly.

You just swore at a guy who cut you off on the way to work. Your children have taken to calling you the Wicked Witch. You start daydreaming about cocktail hour shortly after lunch. And your mind always seems to be in a whirl. What's your problem?

In a word: stress.

You're not alone. Nearly half of all Americans are concerned about the level of stress in their lives, according to the American Psychological Association's 2006 Stress Survey.

Chalk it up to "our overscheduled, harried 21st-century lifestyle, which can wreak havoc with our relationships and our work," says Bruce S. McEwen, M.D., a coauthor of The End of Stress as We Know It (Joseph Henry, $20, amazon.com).

Not to mention our health. Chronic stress weakens the immune system and increases the risk for a range of illnesses, including heart disease and depression.

Stress drives people to eat too much, sleep too little, skimp on exercise, and shortchange fun. It doesn't have to be toxic; a little stress can sharpen focus, improve memory, and heighten emotions.

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In a matter of seconds and without even thinking, you begin pumping out brain chemicals and hormones, including adrenaline and cortisol.

Your heart rate accelerates, oxygen-bearing red blood cells flood the bloodstream, the immune system gears up for the possibility of injury, and energy resources are diverted to your muscles, brain, heart, and lungs and away from functions, such as digestion and hunger, that can wait until the crisis has passed.

You're ready for action, whether it's a full-out battle or a hasty retreat in this case, fleeing back onto the sidewalk to escape the speeding bus.

When the danger has passed, all these systems are restored to their normal resting state. "Your stress response makes you get out of danger," says Sternberg. "Without it, you'd be dead."

Many of the physical changes that energize you to get out of the way of the bus are the same ones at work in more positive situations. Your heart races when you're falling in love.

Your palms grow sweaty on the first day of a new job. How do you know if it's happiness or stress? You label what you're feeling: Your racing heart and sweaty palms come first, and whether you perceive yourself to be excited or happy or stressed depends on your assessment of the circumstances. In other words, you're not running because you're scared; you're scared because you're running.

"Our stress isn't a result of the event but of the view we take of it," says Ellen J. Langer, a professor of psychology at Harvard University. "If the event is seen as negative, then we've set ourselves up to experience stress." RealSimple.com: Easy ways to unwind

Things go awry when stress persists and your body has no opportunity to return to its resting state.

Escape the oncoming bus and you're obviously out of danger. Calm returns both physiologically and emotionally. Your pulse slows, and the panic subsides. No problem.

But getting back to normal is much more complicated when the source of your stress is constant and negative and when fighting or fleeing is not an option: when you're constantly worrying about not having enough money to retire, for example, or when you're trapped in an unfulfilling job.

"The human mind is so powerful, the connections between perception and physiology so strong," says Bruce S. McEwen, M.D., "that we can set off a stress response by just imagining ourselves in a confrontation with the boss."

Emotional stressors such as frustration with a job or a relationship can accumulate until the fight-or-flight response is constantly ramping up. "We become overloaded," says McEwen, who is also a professor of neuroendocrinology at Rockefeller University, in New York City.

"As a result, we begin to feel tired, edgy, and rundown. Eventually this state veers toward illness." Unless, that is, you put the brakes on from time to time. McEwen and his colleagues found that rats kept under significant stress for five weeks developed a seriously impaired immune response.

But if the stress was alleviated for one week, the immune system began to bounce back.

Who is most at risk?

"There's no doubt that the physiology that fuels a chronic stress response can make us sick," says Esther M. Sternberg, M.D. But it doesn't "cause" heart attacks or depression in the way a virus causes the flu.

How your body responds to stress depends in part on your genes. Take an autoimmune disease like rheumatoid arthritis. "There are 20 different genes on 15 different chromosomes that determine whether or not a person will get inflammatory arthritis," says Sternberg. "If you have all 20 genes, you'll most likely get it. If you have two, you probably won't. If you have five, you may or may not get the disease, and the degree to which your stress response is activated could tip the balance."

Your physiological destiny isn't completely fixed, however.

Although we're programmed to develop in a certain way and at certain times, early stressful experiences, such as the death of a parent, can alter the way our brains develop and actually change them. "We now know that when children are exposed to a stressor early in life, perhaps even before birth, their immune and endocrine systems are affected," says Bruce S. McEwen, M.D.

As a result, those people tend to be more sensitive to stressors later in life and are more likely to suffer the health consequences.

Stress and your immune system

The dual nature of stress, both protective and destructive, is mirrored in the dual role of cortisol, a hormone central to fight or flight. In an emergency, a jolt of physiological chemicals, including cortisol, boosts your immune response -- a good thing.

Once the emergency has passed, however, it is also cortisol's job to inform the immune system to return to normal. If too much stress keeps excess cortisol circulating for too long, your immune cells become sluggish and eventually die, leaving you open to infection.

In some people, the body is not able to produce enough cortisol. In these cases, the immune response runs amok, attacking friend and foe alike, eventually taking aim at the body's own cells.

The result can be autoimmune disease, allergies, chronic fatigue syndrome, or asthma.

Cortisol also directs where your body stores fat, planting more on the belly, where it increases your risk of diabetes and heart disease.

This is especially troublesome for women, who, if not for stress, would probably deposit fat on the hips and the thighs, where it's less harmful.

What about the flip side? If too much stress makes you sick, can minimizing stress help you heal? Maybe. A growing number of medical facilities are approaching this question through integrative programs that help patients control illnesses by controlling stress.

One of the oldest is the Center for Mindfulness, at the University of Massachusetts Medical School, in Worcester. "We teach patients mindfulness practices, including mindful breathing, yoga, and meditation," says Melissa Blacker, a codirector of professional education and training at the center.

"Our research suggests that the training can be helpful for people managing conditions like psoriasis, recurrent prostate cancer, and even hot flashes."

You can control stress

If you feel chronically overwhelmed, try shifting your perspective. Rather than seeing stress as something on the outside pushing in, see yourself pushing it away, and consider these proven ways to get out from under its grip.

• Take charge: "When I ask general audiences if they can control their stress level to make it work for them, no more than half say they can," says Esther M. Sternberg, M.D. "If I ask audiences of pilots or neurosurgeons the same question, they all say they can."

The difference is the amount of control these people believe they have over the situation. Pilots, neurosurgeons, firefighters, and others with high- pressure occupations are trained to use the stress response as a way to monitor their own behavior.

When a pilot flies a plane through a storm, his heart races, his breath becomes shallow, and his attention is intensely focused on the job at hand. He experiences the physiological arousal that defines stress, but he doesn't label the situation as stressful. He's done this before. He knows what to expect. He's in control.

On the other hand, if you're a passenger in the airplane cabin, you're uncomfortable because the plane is bouncing around and you can't do anything about it. You feel stressed. "The trick in these kinds of situations is to make yourself feel, little by little, as though you're in control, to make an unknowable situation seem knowable," Sternberg says.

Watch the flight attendant. He has the experience to recognize how much danger the plane is in. If he looks calm (and he probably does), you'll feel calmer.

If your work schedule sets your teeth to grinding, make a list of projects you need to get done and front-load it with tasks you can accomplish quickly. As you check off accomplishments, you'll begin to feel in control, and your stress will ease.

Try scheduling daily chores so that you can attack the most difficult ones when your energy level is highest. And delegate. Not just to coworkers but also to your children, your spouse, and your friends.

• Give yourself time-outs: Of course, you can't control everything. Your child's schedule will inevitably conflict with your work deadlines.

Bad weather will flood the picnic. "We know that chronic stress has a physical impact on your body," Esther M. Sternberg, M.D., says, but if you interrupt stressful moments with calm ones, "you can lessen that impact."

And it's easier than you think to get positive results. If you have a series of crushing deadlines looming at work, take some R&R in between them. A weekend at the beach can restore your equilibrium.

Distract yourself with something you find soothing: Cook, knit, or break out the watercolors. And if you can't take off for the afternoon when you feel your stress rising at the office, just get out for a walk. Even a short stroll can make a difference.

• Reach out, don't retreat: For women, fight or flight should probably be called "tend or befriend," because their response to stress is less about fighting or fleeing and more about turning to family and friends, according to Shelley E. Taylor, a professor of psychology at UCLA, and her colleagues.

Women have more intimate social networks than men do, and when they're stressed, they turn to these networks for support. They're more likely to seek out the company of other women and less likely to flee or to fight.

A stress response moderated by a system such as tend or befriend, as Taylor reported in the July 2000 Psychological Review, might help explain why women live five years longer, on average, than men.

So indulge in the pleasure of family and friends. Invite old pals to a women's night out. Call your sister. Recent studies show that Americans are feeling more isolated; try to fight that situation. It may help you live longer.

• Eat moderately and keep moving: The same hormones that boost your body's supply of available energy in fending off an impending threat also tell your brain that you need to replenish that energy once it's used up.

The result: Your cortisol-crazed psyche sends you on the prowl for all-too-fattening pizza, potato chips, and ice cream in an effort to refuel quickly. If you're going through a stressful period, fight the urge to snack endlessly. Preferably, eat small low-carb, low-fat meals.

And try to exercise regularly.

Working out counteracts the unhealthy buildup of body fat and dissipates the nervous energy that often drives you to that carton of ice cream. The exercise doesn't have to be strenuous.

Walking releases endorphins that can soothe a jangled mind. And even half an hour a day can ease insomnia, which is both a symptom of stress and a stressor.

• Practice mindful relaxation: Physiologically, relaxation is the opposite of stress. When you're relaxed, your breathing and heart rate slow and your mind clears. Mindfulness is a way to achieve this level of relaxation using a variety of techniques, including yoga, meditation, and simple relaxation exercises.

Mindfulness quiets your chattering mind by teaching you how to observe your thoughts and feelings without seeing them as positive or negative. It trains you to use your breathing and an awareness of your body to focus on the here and now.

The basic relaxation response was first described in 1975 by Harvard Medical School researcher Herbert Benson.

His approach has two steps: First, close your eyes and focus on your breath (that's the foundation). Second, choose a phrase, a word, or a prayer and repeat it to stay in the moment and be mindful. "I use two phrases," says Bernadette Johnson, director of integrative medicine at Greenwich Hospital, in Connecticut. "'I'm breathing in relaxation and peace' when I inhale. 'I'm breathing out tension and anxiety' on the exhale."

Ideally, you'd begin and end your day with 10 to 20 minutes of regular relaxation exercise. But should you find your tension rising during the day, "take a deep breath, hold it for a count of four, and exhale for a count of four," Johnson says. "That's what we call a mini, and if it's built on a foundation of regular, longer relaxation exercises, you can tap into it whenever you need it."

If a thought or an emotion intrudes on your mindfulness and threatens to take you out of the moment, observe it but don't react to it.

Think of it as a leaf floating by on a slow-moving stream:

Why did I snap at my husband? Inhale. Let it glide on by.I'm so afraid I'm not going to finish that report in time.Exhale. Ah, another leaf in the swirling current. I'm so stressed-out. Not anymore.